Friday, April 12, 2013

Radon

The largest source of natural background radiation is radon gas. In Chapter 17 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss radon.
“The naturally occurring radioactive nuclei are either produced continuously by cosmic γ ray bombardment, or they are the products in a decay chain from a nucleus whose half-life is comparable to the age of the earth. Otherwise they would have already decayed. There are three naturally occurring radioactive decay chains near the high-Z end of the periodic table. One of these is the decay products from 238U, shown in Fig. 17.27. The halflife of 238U is 4.5 × 109 yr, which is about the same as the age of the earth. A series of α and β decays lead to radium, 226Ra, which undergoes α decay with a half-life of 1620 yr to radon, 222Rn.

Uranium, and therefore radium and radon, are present in most rocks and soil. Radon, a noble gas, percolates through grainy rocks and soil and enters the air and water in different concentrations. Although radon is a noble gas, its decay products have different chemical properties and attach to dust or aerosol droplets which can collect in the lungs. High levels of radon products in the lungs have been shown by both epidemiological studies of uranium miners and by animal studies to cause lung cancer” 
In Chapter 16 we consider radon in the context of the risk of the general population to low levels of background radiation.
“The question of a hormetic effect or a threshold effect [as opposed to the linear no-threshold model of radiation exposure] has received a great deal of attention for the case of radon, where remediation at fairly low radon levels has been proposed. Radon is produced naturally in many types of rock. It is a noble gas, but its radioactive decay products can become lodged in the lung. An excess of lung cancer has been well documented in uranium miners, who have been exposed to fairly high radon concentrations as well as high dust levels and tobacco smoke. Radon at lower concentrations seeps from the soil into buildings and contributes up to 55% of the exposure to the general population.”
Given the high profile of radon in our book, I thought readers might be interested in a bit of the history of this element. A brief discussion of the discovery of radon can be found in Isaac Asimov’s book Building Blocks of the Universe. After Asimov describes the Curies' discoveries of radium and polonium from uranium ore in the late 1890’s, he writes
“When the radium atom breaks up, it forms an atom of radon, element No. 86. Radon is a gas, a radioactive gas! It fits into the inert gas column of the periodic table, right under xenon, and has all the chemical characteristics of the other inert gases.

Radon was first discovered in 1900 by a chemist named F. E. Dorn, and he called it radium emanation because it emanated from (that is, was given off by) radium. [William] Ramsay and R. Whytlaw-Gray collected the gas in 1908, and they called it niton from a Greek word meaning ‘shining.’ In 1923, though, the official name became ‘radon’ to show that the gas arose from radium…

Other gases arise from the breakdown of thorium and actinium … and have been called thoron and actinon, respectively. These are, as it turns out, varieties [isotopes] of radon. However, there have been suggestions that the element be named emanon (from ‘emanation’) since it does not arise from the breakdown of radium only, as ‘radon’ implies.”
Asimov’s Biographical Encyclopedia of Science and Technology describes the scientist who discovered radon in more detail.
“Dorn, Friedrich Ernst, German physicist
Born: Guttstadt (now Dobre, Miasto, Poland), East Prussia, July 27, 1848
Died: Halle, June 13, 1916

Dorn was educated at the University of Konigsberg and taught physics at the universities of Darmstadt and Halle. He turned to the study of radioactivity in the wake of Madame Curie’s discoveries and in 1900 showed that radium not only produced radioactive radiations, but also gave off a gas that was itself radioactive.

This gas eventually received the name radon and turned out to be the final member of Ramsay’s family of inert gases. It was the first clear-cut demonstration that in the process of giving off radioactive radiation, one element was transmuted (shades of the alchemists!) to another. This concept was carried further by Boltwood and Soddy.”

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